1. Field of the Invention
The present invention relates to an apparatus and a method for detecting phase currents of an inverter, and more particularly, to an apparatus and a method for detecting phase currents of an inverter, which detect three-phase currents of an alternating current (hereinafter referred to as ‘AC’) power that an inverter outputs to a load.
2. Description of the Related Art
In general, an inverter has been widely used in order to control a load such as an induction motor with precision. The inverter generates a pulse width modulation (referred to as ‘PWM’ hereinafter) signal, and selectively switches a plurality of switching elements by the generated PWM signal. According to switching of the switching elements, a direct current (referred to as ‘DC’) power is converted to an AC power, and the converted AC power is supplied to a load, thereby driving it. An Insulated Gate Bipolar Transistor (IGBT) has been widely employed as an example of the switching elements.
As a pulse width of the PWM signal varies, switching times of the plurality of switching elements, a voltage and a frequency of the AC power supplied to the load change to thereby control a toque and a rotating speed of the load with precision.
Further, in response to a command signal of the load, a sine wave voltage having a predetermined frequency is generated as a threshold voltage, and a triangular wave having a predetermined time period is generated. The generated triangular wave is compared with the generated sine wave in level, and a PWM signal is generated according to the comparison result. A time period of the PWM signal is identical with that of the triangular wave. A pulse width of the PWM signal varies according to a difference between a voltage level of the triangular wave and a voltage level of the sine wave.
When such an inverter supplies an over current to the load, the load is damaged, and components constituting the inverter are also broken due to the over current. Accordingly, the inverter detects three-phase currents supplied to the load, and judges whether or not an over current is supplied to the load based on the detected three-phase currents. When it is judged that the over current is supplied to the load, an operation of the inverter stops to prevent the load and the inverter from being damaged due to the over current.
There are known four methods to detect three-phase currents that the inverter supplies to the load. A first method is a method of inserting a plurality of current sensors such as a current transformer between an output terminal of an inverter and a load. A second method is a method of inserting a plurality of current detecting resistors between an output terminal of an inverter and a load. A third method is a method of inserting one current detecting resistor between a negative terminal of a power converter for supplying a DC power and a ground of a switch section. A fourth method is a method of inserting respective current detecting resistors between a plurality of lower switching elements switching a flow of an electric current from a load to a ground and the ground, respectively.
The first method has a disadvantage that the sensors themselves are manufactured at very high cost.
In the second method, a difference of a voltage that a switch section of the inverter supplies to the load and a voltage of a controller of controlling an operation of the inverter is great. For this reason, it requires insulation means to insulate a control circuit from the switch section. The insulation means is expensive and a circuit is complex in construction.
The third method has an advantage that a circuit is simple in construction. However, in order to calculate all the three-phase currents, one current detecting resistor should frequently correct a voltage of a sine wave, which leads to a significantly complicate control.
The fourth method can detect an electric current by a single control. According to the fourth method, so as to detect three-phase currents that the inverter outputs to the load, it must be able to detect at least two phase currents.
That is, the three-phase currents supplied from the inverter to the load are expressed by an equation (1).IA+IB+IC=0  (1)
Where, IA, IB, and IC are A-phase, B-phase, and C-phase currents.
As illustrated in equation, because a sum of the three-phase currents is zero ampere, at least two of the three-phase currents are detected, and a remaining one phase current can be calculated based on the two detected phase currents.
So as to detect the three-phase currents supplied from the inverter to the load, a current loop should be constructed to include a plurality of current detecting resistors. In order to measure voltages across the plurality of current detecting resistors in the current loop, a minimal pulse width of the PWM signal is required to flow the phase currents to respective current detecting resistors. A minimal time required to flow a phase current to the respective current detecting resistors depends on the minimal pulse width of the PWM signal. Accordingly, when the minimal time required to detect phase currents in the current loop is insufficient, the phase currents supplied to the load cannot be detected.
While the phase currents are not detected, when an over current from the inverter is supplied to the load, the over current cannot be detected, and components constituting the inverter and the load are damaged due to the over current.
Accordingly, even when each pulse width of two PWM signals is narrower than a minimal pulse width, a detection of the three-phase currents supplied to the load is required.